It is well known that a gas, at relatively low pressures and temperatures, conforms well to the ideal gas law. This gas law is PV=nRT and is also referred to as the Van der Waal gas equation. In this equation, P is the pressure, V is the volume, n is the number of moles of the gas, R is the Rydberg or universal gas constant and T is the temperature. This formula of course can be rearranged to yield
            n      ⁢              R        V              =          p      T        ,where the left side of the equation is the molar density of the system. This is useful because one can see that if the volume of a cavity does not change, it is possible to detect a leak in the cavity based on the simple ratio of pressure over temperature. This concept has been used in the past, and there are a number of patents which are assigned to Kulite Semiconductor Products, Inc., the assignee herein which relate to such apparatus. See for example, U.S. Pat. No. 4,766,763 issued on Aug. 30, 1988 entitled “Gas Leak Detection Apparatus and Methods” by A. D. Kurtz the inventor herein. That patent discloses a gas leak detector employing pressure transducers. In one embodiment the pressure in a vessel is monitored by means of a pressure transducer. The output of the pressure transducer is coupled to an operational amplifier, where the amplifier has a gain which is proportional to
  1  Tabsolute. Thus, since the gain is inversely proportional to temperature, the amplifier produces an output which is independent of temperature but which is capable of providing a pressure indication when there is a gas leak in the container. See also U.S. Pat. No. 5,428,985 issued on Jul. 4, 1995 to A. D. Kurtz et al., and assigned to the assignee herein. That patent is also entitled “Gas Leak Detection Apparatus and Methods” and shows an improved gas leak detection apparatus for detecting a leak in a vessel. The apparatus uses a pressure transducer, an amplifier and feedback which operates to effectively and accurately model the VanderWaal equation of gases. The apparatus is adaptable for operation with any number of different gases by simply changing the values of specific circuit elements. In that patent there is also disclosed the above noted formulas as well as modifications of the same. See also U.S. Pat. No. 6,935,184 issued Aug. 30, 2005, to A. D. Kurtz and entitled “Pressure Transducer Capable of Detecting Internal Leakage of External Media”. That patent describes a pressure transducer which has a first header containing a first pressure sensor for measuring pressure of the media. A second header contains a second pressure sensor and is disposed at the second end of a spacer. The spacer and the header assembly is formed with a sealed leak detection cavity for capturing the media that leaks past the first header.
Thus, it is apparent that using pressure transducers to detect leaks in various containers and vessels is well known as indicated by the above noted materials. The present invention is a circuit which produces an output which is proportional to pressure over temperature for use in leak detection and situations with relatively low pressures and temperature such as those pressures found in an automobile tire or other chamber.